1. Field of the Invention
This invention relates generally to a radiation imaging sensor, and more particularly to a radiation imaging sensor having high sensitivity, wide latitude and high spatial resolution.
2. Description of the Prior Art
Computed Radiography apparatuses using a photo-conductive material (CR apparatuses) are known in the art and their structure is discussed in SPIE, Vol. 173 (1979), pp. 81-87 and in Japanese Patent Laid-Open No. 228373/1986, for example.
One of the problems with the conventional computed radiography apparatus is that a sufficiently high S/N cannot be obtained. Hereinafter, S/N of a typical sensor will be discussed so as to clarify the problem with the prior art technique.
X-rays absorbed by a phosphorescent material is typically about 50%, though the quantity varies depending on the constituent material and the thickness of the phosphorescent layer. The number of photons L generated at this time is given by: EQU L (number of photons/pixel)=N.sub.x x E/W x S x A (1)
where N.sub.x is the number of X-ray photons, E is X-ray energy, W is the W value of the phosphorescent material, S is a pixel size and A is an X-ray absorption factor of the phosphorescent material.
It will be hereby assumed that they have the following values, respectively: EQU N.sub.X (cm.sup.-2).about.4.times.10.sup.4 R (.mu.R) (R: X-ray dose), EQU E=50.times.10.sup.3 eV, EQU W=15 eV, EQU S=(100 .mu.m).sup.2= 10.sup.-4 cm.sup.2, EQU A=0.5.
Then, L is given as follows: ##EQU1## Assuming that the X-ray dose incident to the sensor is 10 (.mu.R)&lt;R&lt;10 (mR), then, the following relation is established from eq. (2): EQU 6.7.times.10.sup.4&lt; L (number of photons/pixel)&lt;6.7.times.10.sup.7 ( 3).
In other words, the photoconductive layer must detect such weak light per pixel.
The charge Q obtained by incidence of light is given by the following equation: EQU Q=L.times.k.times..eta..times.1.6.times.10.sup.-19 (C) 4
where K is efficiency of incidence of light into the photoconductive layer and .eta. is quantum efficiency of photoelectric conversion and .eta..ltoreq.1.0 when no propagation exists. If this charge is read out in .tau. seconds per pixel, the resulting signal current S is given by: EQU S(A)=Q/.tau. 5
Assuming that k=1.0, .eta.=1.0 and hence .tau.=1.times.10.sup.-5 S, then the signal quantity S for L of eq. (3) becomes a weak current as expressed by the relation below: EQU 1.1.times.10.sup.-9&lt; S(A)&lt;1.1.times.10.sup.-6 6
Therefore, signal detection is extremely difficult. Furthermore, since the absolute value of its signal component is small, S/N drops at the time of signal detection, so that picture quality deteriorates in the image having a small radiation dose.
On the other hand, the increase in optical detection sensitivity has been reported by avalanche multiplication inside a blocking type photoconductive film as a result of studies of imaging tubes (ITEJ Technical Report, Vol. 10, No. 45, p.p. 1-6, 1987). However, this technique cannot be utilized as such for radiation measurement because the stopping power of the photo-conductive film for X-rays is low. If the thickness of the photoconductive film is merely increased in order to improve X-ray stopping power, problems develop such that stable propagation cannot be made and spatial resolution drops during the propagation process.